I'm having an issue with my chain shifting when I'm not changing gears on my MTB. I wasn't having any problem when pedaling for 10 minutes with the left grip shifter on "3" and the right grip shifter on "5." Then all of a sudden I felt a change in my pedaling (I would describe it as a slip of the chain for a half a second). I'm not sure how to describe it better but it is strange having my ride be normal and then hearing and feeling a change for a split second when pedaling. Any idea what this is and is it normal? I recall having the same issue on a MTB that I had as a kid, but this bike that I bought just received a tune-up, so I thought I wouldn't be having this experience.